The present invention relates to electric motor control centers and the like, and more particularly to an improved construction for an enclosure which receives a removable contactor carriage.
It is conventional for equipment used for stopping, starting, and controlling the operation of large electric motors to be housed in sheet metal cabinets. Such apparatus ordinarily includes a contactor which opens and closes the main load circuit, and auxiliary control equipment such as cutout relays, current transformers, overload relays, etc. Such equipment is usually intended to both protect electric motors against short circuit or overcurrent conditions, while allowing large currents to flow during startup and temporary overload conditions. The actual apparatus which opens and closes the load circuit, and which is therefore called upon to make and break large currents, is usually contained in a separate cubicle which is physically isolated from other compartments which contain control units which typically operate at much lower voltage levels.
Both because of its size and because of the need for inspection and repair, the large, heavy electromechanical contactors are conventionally incorporated into a removable unit, herein termed a carriage. In many cases the carriages are provided with wheels so they may be rolled in and out of their compartment; in other cases rollers are provided within the compartment to ease the egress of the contactor carriage. The carriage is provided with means for engaging the large, heavy conductors which conduct current from the supply line to the contactor and from the contactor to a load. Due to the heavy currents involved, the engaging elements are necessarily of substantial construction and ordinarily take the form of a female element such as a set of spring-loaded fingers attached to the contactor carriage and a male element, termed a "stab" and ordinarily formed of a heavy copper bar, which extends from the base or back wall of the compartment. Since most heavy electrical installations utilize three-phase power, a total of six sets of stabs and fingers are provided. Due to their necessary rigidity and size, they must be maintained in precise alignment with one another. They must also be properly aligned with respect to the contactor carriage so that all connections are made properly, without gaps which could give rise to arcing.
For the foregoing reasons it is necessary that the means which locate and engage the contactor carriage be of the highest structural integrity, since a small amount of flexure could result in substantial misalignment and make it difficult or impossible to properly install or "rack" the carriage assembly.
Due to the relatively large size of the cabinets which contain the motor control center equipment a good deal of deformation and misalignment necessarily occurs if the cabinets are not handled with some care and mounted upon flat, level surfaces. In order to provide the structures with sufficient rigidity to maintain their internal dimensions, the framework is often fabricated of heavy structural members which are welded together into a monolithic frame, then covered with sheet metal panels. This approach results in a structure which is a good deal heavier and more expensive than necessary for supporting the cabinet during normal use. In addition even with heavy, rugged construction the cabinet may be badly distorted by improper lifting or handling, or installation upon uneven surfaces. It will therefore be appreciated that it would be highly beneficial to provide a form of motor control center construction which accommodates an economical structural framework susceptible to some deformation and misalignment but adequately strong for in-place support, while maintaining the structural relationships among the removable, interengaging high-current elements within the cabinet.
It is therefore an object of the invention to provide a motor control center construction which tolerates substantial structural deformation and misalignment yet allows the ready engagement and disengagement of a contactor carriage assembly.
Another object is to provide rigid means for locating a contactor carriage assembly within a motor control center compartment whose geometry is substantially unaffected by common forms of deformation and misalignment of the enclosing structure.
Yet another object is to provide rigid, independent structure within a motor control center cabinet which maintains the relative alignment among current engaging elements, contactor carriage assembly, and racking mechanism.
Yet another object of the invention is to provide a separable structure to be carried within a motor control center compartment which provides for the proper location of interlock and racking mechanisms and current-carrying elements with respect to a separable contactor carriage assembly.